James Mattis says the UK needs to invest and maintain robust military capability.
Photograph: Matt Dunham/AP

The US defence secretary, James Mattis, has made a surprise intervention in the row over UK defence spending by warning France could replace the UK as Washington’s closest military ally in Europe.

In a leaked letter to the British defence secretary, Gavin Williamson, Mattis expressed concern the UK’s military power was at risk of erosion and compared the UK unfavourably with France, which Mattis pointed out had committed to significant increases in defence spending.

“As global actors, France and the US have concluded that now is the time to significantly increase our investment in defense. Other allies are following suit,” he said.

“It is in the best interest of both our nations for the UK to remain the US partner of choice.”

The letter was sent to Williamson on 12 June, three days after a visit by Mattis to London. Williamson would be unlikely to have leaked it without first seeking approval from his American counterpart.

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If Mattis sent the letter to help Williamson in his campaign for more money, it marks an unusual departure for a senior US politician, who would normally not intervene in British domestic politics in such a public way.

One of parliament’s most outspoken advocates of funding for the armed forces will criticise Williamson’s tactics in a speech on Tuesday, warning the debate around spending is at risk of becoming debased by negative briefing.

The Conservative MP Jonny Mercer, a former British army officer and member of the defence select committee, will say in a speech at the Royal United Services Institute in London that the “cacophony of newspaper headlines, blackmail and bullying around defence spending is in danger of pushing an extremely serious subject into the absurd”.

“The way some of this has been played out in the public domain has been entirely misinformed, dangerously undermining of any logical and opinion-forming strategy that could win support for more funding, and frankly embarrassing for all concerned,” he will say.

However, Mercer’s speech will also take aim at the government, which he will say is failing to grasp the gravity of the funding issue, and of the strength of feeling around the prosecution of troops for historical offences.

He will warn the UK has “fundamentally changed in the eyes of our adversaries because of underfunding … and our standing internationally is no doubt the poorer for it.”

Donald Trump is pressing all members of Nato to spend more on defence and will make the plea in person at a Nato summit in Brussels next week.

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The UK meets the Nato target of spending 2% of GDP on defence, but Theresa May and the chancellor, Philip Hammond, are ruling out any significant increases in spite of pressure from Williamson.

France spends 1.7% of GDP on defence, below the Nato target, but the country’s president, Emmanuel Macron, who is aiming to meet the goal, announced in February that billions more would be spent through to 2025.

Since Trump became president in January 2017, French diplomats and the military have been watching with interest a divergence between London and Washington on a host of issues, raising hopes that it might yet supplant the UK.

The period is in contrast with 2003, when relations between Paris and Washington were strained because France declined to join the US in invading Iraq.

Mattis argued in the letter: “I am concerned that your ability to continue to provide this critical military foundation for diplomatic success is at risk of erosion, while together we face a world awash with change.”

Mattis said that while it is in the best interests of the US and UK that Britain remains the partner of choice, “the UK will need to invest and maintain robust military capability”.

“It is not for me to tell you how to prioritize your domestic spending priorities, but I hope the UK will soon be able to share with us a clear and fully funded forward defence blueprint that will allow me to plan our own future engagement with you from a position of strength and confidence,” he said.

“A global nation like the UK, with interests and commitments around the world, will require a level of defence spending beyond what we would expect from allies with only regional interests. Absent a vibrant military arm, world peace and stability would be at further risk.”

Mattis’s position in the Trump administration is under threat and he has been sidelined on recent key issues.

The UK government is struggling to finalise its defence review before the Nato summit as promised. Unable to agree on more spending, the choice is increasingly between publishing a vague statement of intent or not publishing anything.

A Ministry of Defence spokesman said: “The UK maintains the biggest defence budget in Europe and we have been clear we will continue to exceed Nato’s 2% spending target.